Boston Police Commissioner William G. Gross was sworn in four months ago to great fanfare, promising to improve relations between the community and the department.

But that pledge is being tested after he denounced the American Civil Liberties Union over the weekend on his personal Facebook page as “paper warriors” more concerned with filing lawsuits than helping police combat crime.

The unusual move drew a sharp response from the civil rights group, which recently filed a lawsuit that accused Boston police officers of unfairly labeling Central American teenagers as gang members, making them more vulnerable to deportation. On Sunday, Carol Rose, the group’s executive director, said Gross was trying “to divert attention from the serious issues raised by an ACLU lawsuit.”

“As a police commissioner representing about 2,000 uniformed officers, Commissioner Gross recognizes the significance of any comment he makes both in his professional capacity and in his personal life,” said Sergeant Detective John Boyle. He “will continue to make protecting and serving all residents and communities as well as our officers his top priority.”

In a statement, Walsh, who made history last summer when he named Gross the city’s first black commissioner, expressed support for Gross.

“I have made clear that the residents of the City of Boston deserve to always feel safe and protected, regardless of their background or status,” Walsh said. “What is most important to me is that we have a police commissioner who prioritizes the safety of our residents above all else, which is exactly what Commissioner Gross does every single day.”

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But Gross’s remarks prompted pushback from other political leaders and seemed to reflect a more hardline stance from a law enforcement official who had been widely viewed as friendly and open to change.

“There are some legitimate concerns raised in the lawsuit that we as a city have to address and the best way to do that is be in discussion and at the table together,” Andrea J. Campbell, president of the Boston City Council, said in a statement. “While both our police department and the ACLU do incredible work, we have to recognize where we can do better and that requires us as public officials not to take a defensive posture.”

Nika Elugardo, who was elected state representative in November, agreed.

“We’ve got defensiveness coming from our political leadership because they’re aware of how much they’re doing and it’s difficult for them to hear the critique,” said Elugardo, whose district represents parts of the city that are heavily Latino and have been affected by violence. “It is critical for our top leaders to understand that we need to lay that defensiveness down or we’re not going to get anywhere.”

The remarks from Gross, who has kept a low profile as commissioner and is known more for doling out hugs than criticism, were also surprising given that his predecessors rarely feuded publicly with the ACLU, which has a long history of suing police departments for alleged civil rights violations.

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Police leaders said his comments reflected frustration with civil rights advocates who refuse to recognize the city’s reduced crime rates or the need for police to develop and share gang intelligence to keep residents safe.

Michael F. Leary, president of the Boston Police Patrolmen’s Association, said his phone had been flooded with texts and calls from officers thrilled by Gross’s statements.

“The guys are behind him 100 percent,” Leary said. “The ACLU is trying to bog us down with lawsuits and we’re trying to do the right thing. No one is trying to target anyone. We’re trying to target criminals.”

Michael Talbot, vice president of governmental affairs for the Massachusetts Fraternal Order of Police, said it has been infuriating for police to be criticized for using surveillance tactics that have helped solve crimes like the Boston Marathon bombings.

Gross has “done investigations his whole life and he understands that these are important tools for solving crimes,” Talbot said.

“NO ACLU when Officers are shot, No ACLU when we help citizens, no ACLU present when we have to explain to a mother that her son or daughter was horribly murdered by gang violence,” Gross wrote in Saturday’s post.

In the post, Gross said the ACLU knows nothing of telling a mother her child has been killed and has never expressed sympathy for an officer shot in the line of duty.

“They are never in the streets,” Gross wrote. “I sure as hell didn’t see the ACLU in El Salvador working to find a solution to our youth being inducted into the MS-13 Gang and The 18th Street Gang.”

Daniel F. Conley, the former Suffolk district attorney, said Gross’s affable personality belies a fierce devotion for the department and public safety.

“One of the attributes I love most about Willie is his authenticity,” Conley said. “I don’t think he was necessarily being defensive. I think there is a part in him that says ‘Can’t they see that we are honorable people and do good and are honorable in the discharge of our duties?’ ”

The ACLU of Massachusetts is one of several plaintiffs in the lawsuit, which is seeking to compel Boston police to release details about the department’s practice of identifying young men and teenagers as gang members and making the information available to federal immigration officials.

Tanisha Sullivan, president of the Boston chapter of the NAACP, which is not involved in the lawsuit, said that the ACLU is seeking demographic data that other police departments have shared with the public.

“Our experience with the ACLU, and that of so many other civil rights organizations, is that they have been working on the ground and working vigorously not only on civil liberties issues as we know them, but to advance racial justice and women’s rights,” Sullivan said.

Boston City Councilor Ayanna Pressley, who was just elected to Congress, said she supported the ACLU’s efforts to learn more about the database.

“I share the serious concerns raised by the ACLU, and several other civil rights organizations, about the potential profiling of Boston residents — specifically in communities of color — and the targeting of undocumented immigrants as a result of the BPD’s gang database,” she said.